[ddevault]

I'm not sure what you mean by "select by type". I can say that you can define new types:

   type a = int;
   type b = int;
   type c = (a | b);

Which seems to be what you're angling for here? I also suspect that you're approaching this from a Rust- or Zig-like background where enums and tagged unions are closely related; this is not so in Hare.

[erk__]

I do know those languages, but this is more the ADT way from ML.

The example you linked did not really make it clear from my point of view because one of the points is that it will collapse if there are multiples of a specific type. But your way seems to make it possible.

[nyanpasu64]

Tagged unions of free-floating types (polymorphic variants, C++ std::variant) and enum holding cases namespaced within themselves (Rust/Haskell) are two alternative designs. I personally prefer having both in a language. But if only one is available in a language, I prefer free-floating types, since it's more flexible and allows using the same type in multiple tagged unions, and you can somewhat emulate enums using tagged unions and namespaces (like I've done in C++ at https://gitlab.com/exotracker/exotracker-cpp/-/blob/eb8458b2...). If you take that approach, to prevent collapsing in the generic case when two type parameters are the same, you'd have to define newtypes for each type stored in a union.

[ddevault]

Right - but defining a type alias makes a new type, which does not collapse.